to aid viewing the raw markdown source in e.g. a gitlab source browser,
or to aid navigation in vim with "gf" style commands to jump between
files, allow the markdown source to specify foo.md or whatever/index.md
explicitly, yet generate the clean URLs for linking in the HTML output
this assumes that nginx is serving "foo" with foo.html, and "bar/" with
bar/index.html
Signed-off-by: Brian S. Stephan <bss@incorporeal.org>
